I feel like my opk's aren't getting darker as the days go on I'm pretty confused since this is my first time taking them

I'm supposed to have been ovulating already we've been doing the deed every other day just to make sure I'm not missing my ovulation days, but my tests are still faint I know it's supposed to be the same color when it's time. Maybe I ovulate a bit later than the "norm" ? Thanks for any help! 

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